Electrolytic copper foil is an indispensable material for printed circuit board, and especially the double-sided glossy copper foil can be coated with electrode material for the use of cathode plate of lithium ion secondary battery. Following the increasing demands of consumer electronic products, the demands of lithium ion secondary batteries have increased gradually.
In addition, the development trend of commercial small electronic products is moving toward smaller and lightweight designs. The batteries as driving power not only have to possess the advantages of having high operation voltage, high energy density and long cycle life, but they also have to follow the trend of minimization. Since the non-aqueous electrolyte secondary battery, such as lithium ion secondary battery, possesses the features of high energy density, high capacity and long cycle life, thereby, it is used widely as driving power for portable electronic instruments.
In general, the causes of poor preservation of electrolytic copper foil are oxidation and acid corrosion. Therefore, after manufacturing the electrolytic copper foil, it was processed with anti-rust treatment using electroplating, such as chromium coating, or using organic compounds, such as benzotriazole, to form chelate compound with copper, thereby enhancing the resistance of electrolytic copper foil. However, during long term preservation, oxidation and acid corrosion can still occur to the electrolytic copper foil which was processed with electro-plating anti-rust treatment. The defects of poor preservation include the appearance of oxidation spots and discoloring. In the manufacturing process of the copper foils which were used in printed circuit board or the cathode plate of lithium ion secondary battery, the electrolyte is consisting of copper dissolved in sulfuric acid. One possible rationale of causing the defects could be the continuous corrosion of copper foil due to the residual sulfuric acid on the surfaces of copper foil after the formation of foil.
In order to resolve the problem of poor preservation of the electrolytic copper foil due to rusting, in the industry the electrolytic copper foil is washed with water after the anti-rusting treatment using deionized water in two-staged steps to wash the electrolytic copper foil which is coated with anti-rust layer to remove the residual sulfuric ion or other impurities on the surface of the electrolytic copper foil. Also, in the washing steps, there are specific requirements for water pressure, water distribution, washing distance and washing angel. In addition, in a relevant publication, it indicated the high demands of water quality in the washing steps, and in general the deionized water with conductivity less than 10 μS/cm was used (Printed Circuit Communications, volume 10, 2004). In other words, the soft water with good softening processes was used as cleaning solutions. However, this method is effective for a short period of time to prevent the discoloring of the electrolytic copper foil, but it does not achieve significant effects for the long term preservation of the copper foil.
Therefore, it is an urgent need for the industry to develop an effective method for a surface treatment to improve the anti-oxidation and anti-rust corrosion property of the electrolytic copper foil. The method, which should be especially suitable for the washing steps after the anti-rust treatment, should include a cleaning method, a composition of the cleaning fluid to be used for the cleaning method and an electrolytic copper foil resistant to oxidation and rust-corrosion.